The present invention relates to a coated cemented carbide with a binder phase enriched surface zone essentially free of gamma phase comprising WC, a metallic binder based on Co, Ni or Fe and submicron gamma phase.
Cemented carbide grades for metal cutting applications generally contain WC with an average grain size in the range of from about 1 to about 5 μm, gamma phase, a cubic solid solution of at least one of TiC, NbC, TaC, ZrC, HfC and VC, substantial amounts of dissolved WC, and from about 5 to about 15 wt-% binder phase, generally Co. Their properties are optimized by varying the WC grain size, volume fraction of the binder phase and/or the gamma phase, the composition of the gamma phase and by optimizing the carbon content.
The gamma phase increases the hot hardness and also the chemical wear resistance of cemented carbides. It is formed by adding cubic carbides such as NbC, TaC, TiC, ZrC and HfC or mixed carbides of the same elements to a cemented carbide powder. The gamma phase formed during sintering grows by a dissolution and precipitation process and will dissolve substantial amounts of tungsten and will have a grain size of the order of from about 2 to about 4 μm.
US Pat. Appl. Publ. 2005/0126336 discloses a cemented carbide comprising WC, a binder phase based on Co, Ni or Fe and gamma phase in which said gamma-phase has an average grain size of less than about 1 μm. This is accomplished by adding the powders forming gamma phase with a WC-content in equilibrium at a temperature of about 1450° C., a typical sintering temperature, for Ti, Nb and Ta based gamma phase.
Coated cemented carbide inserts with binder phase enriched surface zone are today used to a great extent for machining of steel and stainless materials. Thanks to the binder phase enriched surface zone, an extension of the application area for cutting tool material has been obtained.
Methods or processes to make a cemented carbide containing WC, cubic phase (carbonitride) and binder phase with binder phase enriched surface zones are within the techniques referred to as gradient sintering and are known through a number of patents and patent applications. According to U.S. Pat. No. 4,277,283 and U.S. Pat. No. 4,610,931, nitrogen containing additions are used and sintering takes place in a vacuum whereas according to U.S. Pat. No. 4,548,786 the nitrogen is added in gas phase. The result is that the volume which previously was occupied by the cubic phase after its dissolution is occupied by liquid binder metal. Through this process, a binder phase enriched surface zone is created. The metal components in the dissolved cubic phase diffuse inwardly and are precipitated on available undissolved gamma phase present further in the material. The content of these elements therefore increases in a zone inside the binder phase enriched surface zone at the same time as a corresponding decrease in the binder phase content is obtained. Cracks grow easily in this zone, which has a decisive influence on the fracture frequency during machining. A method of eliminating this problem is disclosed in U.S. Pat. No. 5,761,593.